TN (twisted nematic) liquid crystal cells having a twist angle of 90.degree. have heretofore been used as display terminals of about 1/100 duty cycle, such as watches and electric calculators. In view of the principle, however, it has been believed that it is difficult to increase the number of duty cycles to more than the above value, because it is accompanied by a reduction of display quality.
On the contrary, a supertwisted birefringence effect mode utilizing the birefringence effect of a liquid crystal cell in which the uppermost possible twist angle to produce bistability, one of electro-optic characteristics of a chiral nematic liquid crystal, is controlled between 180.degree. and 270.degree. can be used as a liquid crystal display device of about 1/400 duty cycle. This system is hereinafter referred to as an "SBE mode", including supertwisted nematic modes using an aligning organic film, called an STN mode and an HBE mode.
T. J. Scheffer & J. Nehling disclosed in Appl. Phys. Lett., 45, 1021 (1984) that the above SBE mode can provide a matrix display device to be driven by the same driving manner as that in a matrix display device of the conventional 90.degree. twisted nematic mode, i.e., the fast scan addressing mode responding to the effective value of applied voltage. It is also disclosed that the SBE mode has a much higher contrast and a much wider viewing angle than the 90.degree. TN display mode.
Liquid crystal compositions to be used in liquid crystal display devices of the SBE mode are required to have the characteristics described below.
(1) A voltage-transmittance curve which is steep. This characteristic, i.e., steepness of voltage-transmittance curve is necessary for increasing the display contrast of a liquid crystal display device of the SBE mode.
(2) A nematic-isotropic liquid phase transition temperature (hereinafter referred to as a "clearing point") which is high. In the SBE mode, it is desirable that the clearing point be as high as possible in order to prevent changes in color, due to temperature dependency of refractive index anisotropy.
(3) Viscosity (.eta.) is low. In a liquid crystal cell of the SBE mode, it is desirable that the viscosity be as low as possible, because its response time is in proportion to .eta..multidot.d.sup.2 (wherein d indicates a cell thickness).
Conventional liquid crystal compositions, however, fail to satisfy the above requirements; a liquid crystal composition satisfying the above requirements has not been obtained heretofore.